1. Technical Field
The present invention relates generally to rubber compositions and methods for decreasing the tangent delta value (i.e., hysteresis) and maintaining the abrasion resistance index (i.e., wear resistance). The rubber compositions are particularly useful for tire tread applications in vehicles, e.g., trucks.
2. Description of the Related Art
The tire treads of modern tires must meet performance standards which require a broad range of desirable properties. Generally, three types of performance standards are important in tread compounds. They include good wear resistance, good traction and low rolling resistance. Major tire manufacturers have developed tire tread compounds which provide lower rolling resistance for improved fuel economy and better skid/traction for a safer ride. Thus, rubber compositions suitable for, e.g., tire treads, should exhibit not only desirable strength and elongation, particularly at high temperatures, but also good cracking resistance, good abrasion resistance, desirable skid resistance, low tangent delta values at 60° C. and low frequencies for desirable rolling resistance of the resulting treads. Additionally, a high complex dynamic modulus is necessary for maneuverability and steering control. A long mooney scorch value is further needed for processing safety.
Presently, silica has been added to rubber compositions as a filler to replace some or substantially all of the carbon black filler to improve these properties, e.g., lower rolling resistance. Although more costly than carbon black, the advantages of silica include, for example, improved wet traction, low rolling resistance, etc., with reduced fuel consumption. Indeed, as compared to carbon black, there tends to be a lack of, or at least an insufficient degree of, physical and/or chemical bonding between the silica particles and the rubber to enable the silica to become a reinforcing filler for the rubber thereby giving less strength to the rubber. Therefore, a silica filler system requires the use of coupling agents.
Coupling agents are typically used to enhance the rubber reinforcement characteristics of silica by reacting with both the silica surface and the rubber elastomer molecule. Such coupling agents, for example, may be premixed or pre-reacted with the silica particles or added to the rubber mix during the rubber/silica processing, or mixing, stage. If the coupling agent and silica are added separately to the rubber mix during the rubber/silica processing, or mixing, stage, it is considered that the coupling agent then combines in situ with the silica.
A coupling agent is a bi-functional molecule that will react with the silica at one end thereof and cross-link with the rubber at the other end. In this manner, the reinforcement and strength of the rubber, e.g., the toughness, strength, modulus, tensile and abrasion resistance, are particularly improved. The coupling agent is believed to cover the surface of the silica particle which then hinders the silica from agglomerating with other silica particles. By interfering with the agglomeration process, the dispersion is improved and therefore the wear and fuel consumption are improved.
The use of silica in relatively large proportions for improving various tire properties requires the presence of a sufficient amount of a coupling agent. The silica however retards the cure. Therefore, a silica/coupling agent tread formulation has been found to undesirably slow the cure rate of the rubber. Additionally, by employing high amounts of the coupling agents results in the rubber compositions being more costly since these materials are expensive.
In order to increase the cure rate, secondary accelerators such as, for example, diphenyl guanidine (DPG), have been added to the rubber compositions. However, the use of secondary accelerators, and particularly DPG, result in the rubber compositions having a shorter mooney scorch value during its manufacture thereby resulting in decreased processing time. Problems associated with a decreased processing time include, for example, precured compounds and rough surfaces on extruded parts. Additionally, diphenyl guanidine is typically employed in high amounts which result in the rubber compositions being more expensive to manufacture since more material must be used.
It would therefore be desirable to provide a rubber composition having a decreased cure time and longer mooney scorch value for processing while also a decreased tangent delta value. This will allow for better processing of the rubber composition during its manufacture and an improved resulting tire product.